1. Field of the Invention.
This invention relates in general to the problem of the secrecy of information in communication system in which plural terminal installations receive multiplexed data signal frames and are likely to read in said data signal frames which are not intended for them. More particularly, this invention relates to the encryption of time-division multiplexed bidirectional communications via optical fibers in a telecommunication network e.g. of the ISDN (Integrated Services Digital Network) type.
2. State of the Prior Art.
In an optical fiber telecommunication network, the local servicing of communication signals between a local switching exchange and subscriber installations is carried out by means of a starred service network comprising several transmission optical fibers connecting the switching exchange and the subscriber installations respectively at different ports of a directional optical fiber coupler constituting the central node of the starred service network. Communication between the switching exchange and the subscriber installations is of the collective half-duplex type. During first communication periods, the switching exchange transmits a backward packet towards all the subscriber installations. The backward packet is comprised of plural multiplexed words respectively destined for the subscriber installations. During second communication periods, the subscriber installations transmit words destined for the switching exchange respectively at different determined times. At level of the service network central node, the words transmitted by the subscriber installations are inserted in respective time intervals of a forward packet destined for the switching exchange.
With this type service network there is a problem of secrecy of information in the backward transmission direction from the switching exchange toward the subscriber installations. In fact, each of the subscriber installations receives the entire backward packet and is likely to read in said packet, words that are destined for other installations. In the forward transmission direction from the subscriber installations to the switching exchange, there is no secrecy problem due to the fact that the optical fiber coupler at the center of the network is of the directional type and that a word transmitted by a subscriber installation towards the switching exchange is only received by the latter and may not be received by other installations.